The objectives of this research are (1) to evaluate radiation-induced changes in the immature spinal cord, (2) to use ionizing radiation as a tool to study reparative and regenerative capacities of the immature nervous system, and (3) to study normal postnatal development of the spinal cord. A;; experiments are carried out in rats because the developmental state of these animals at birth makes it possible to study the immature nervous system ex utero. The specific experiments undertaken in this phase of the project will concentrate on (1) the effects of x-rays on occurrence of the reactive (non-neuronal, non-vascular) cells that are observed in the central nervous system following injury to a peripheral nerve, and (2) radiation-induced Schwann cells and peripheral myelin in the spinal cord. The spinal cords are examined light microscopically using histopathologic and autoradiographic methods with both qualitative and quantitative assessments being carried out as needed. The responses of the immature nervous system to injury are compared and contrasted with those reported by others in the adult nervous system; this, perhaps, will provide some insight into the reactions and limitations occurring in the mature state.